The invention dramatically improves the dewatering capability of a twin belt press by the addition of a high molecular weight cationic amine salt to a slurry of solids to be dewatered. The addition of a high molecular weight cationic salt to a feed slurry dramatically improves the capture of solids in the gravity drainage filtrate with much lower solids in the belt wash water and results in a dryer cake at discharge.
Dewatering of slurries, such as waste activated sludge or aerobically digested sludge, is a process of liquid-solid separation wherein large quantities of liquid can be removed from a slurry by mechanical and chemical means. Dewatering does not, however, achieve 100% liquid-solids separation. Normally, moisture remains in a dewatered cake, and in many instances the filtrate contains solids from the slurry. Cake dryness and solids capture are typical indicators of the efficiency of a dewatering system, and are influenced by the mechanical device used, the chemical conditioning of the feed slurry and the characteristic of the slurry itself.
Water in refuse or process slurries may be present in one of three forms: free water, capillary water, or intracellular water. Free water drains easily from the solid particles, since no adhesive or capillary forces are to be overcome. Capillar water can be separated from solids by overcoming the adhesive and capillary forces holding the water amongst the solid particles. Capillary water is typically transformed to free water by increasing the particle size through the use of polymer flocculants. Intracellular water, water contained inside cell walls, can not be removed unlsss all the cell walls are lysed or broken. The breaking of cell walls requires high mechanical forces, heat and/or chemical treatment. Therefore, the content of intracellular water sets the theoretical upper limit for cake dryness in slurries where large amounts of the solids have a biological origin.
The twin belt press is one mechanical device which has been found to be particularly effective in dewatering slurries, such as waste activated sludge or aerobically digested sludge. Although the twin belt filter has been found to be most effective in removing free water from these slurries, it alone is not capable of overcoming the adhesive and capillary forces holding the capillary water in between the solids of the slurry. Due to the desirability of obtaining a much dryer cake at discharge, and reducing the solids permeating into the filtrate, attention has been given to various means for improving the capture of capillary water.
It is well settled that the use of polymer flocculants can assist in overcoming the adhesive and capillary forces holding water between the solids of a slurry, and thereafter transform such capillary water into free water which easily drains from the solid particles in a dewatering device, such as a twin belt press. The use of polymer flocculants in the dewatering of minerals, such as coal, phosphates, slimes, tar sands, mineral tailings, bentonite, and other clay products is demonstrated in the following patents: U.S. Pat. Nos. 3,408,293 (Dajani et al.), 3,578,586 (I. Gal et al.), 4,342,653 (Halverson), and 4,569,768 (McKinley); Japanese Patent Publication No. 49-10182; and European Patent Application No. 81110828.1 (Braun et al.). U.S. Pat. No. 3,408,293, U.S. Pat. No. 4,569,768, Japanese Patent Publication No. 49-10182, and European Patent Application No. 81110828.1 all demonstrate sequential adding of anionic polymers followed by cationic polymers to assist in the flocculation of mineral slurries prior to dewatering. The chemical addition of two polyelectrolytes is undesirable since it dramatically increases the cost of the dewatering process.
U.S. Pat. No. 4,342,653 describes the flocculation of aqueous suspensions of solids, such as phosphates, slimes, tar sands, coal refuse, etc., by addition of a polymeric anionic flocculant.
U.S. Pat. No. 3,578,586 proposes that it is preferable to treat slurries of coal or other minerals with a polyelectrolyte at various points throughout the dewatering system. The polelectrolytes having a molecular weight from 10,000 to 10,000,000 and added in at least two steps.
Ionic flocculants have also been added to slurries of sludge prior to dewatering with a mechanical device. The following patents all demonstrate the use of a two step approach using both anionic and cationic flocculating agents: U.S. Pat. Nos. 4,105,558 (Heinrich et al.), and 4,479,879 (Hashimoto et al.); and Japanese Patent Application Nos. 82/185578, 84/197287, 83/336544, 81/17903 and 81/18499. In particular, Japanese Patent Application No. 82/185578 adds polycations and amphoteric copolymers. Japanese Patent Application No. 83/236544 dewaters sludge by using inorganic coagulants and dewatering agents containing cationic organic polyeer coagulants, anionic organic polymer coagulants and acids. Japanese Patent Application No. 81/18499 mixes the sludge with a cationic polymer coagulant and a surfactant.
There are various patents which describe the use of cationic polymer flocculants to assist in the dewatering of sewage sludge. U.S. Pat. No. 3,531,404 proposes that sludge be mixed with a polyelectrolyte flocculant which has a high molecular weight and is cationic prior to dewatering the sludge. The following patents also disclose the use of a cationic polymer flocculants to assist in the dewatering of sludge: U.S. Pat. No. 4,358,381 (Takeuchi et al.), and Japanese Patent Application Nos. 84/6852,883/226535, 82/98698, 82/69471, 75/148242, 75/148241, 74/146371, 80/43556,778/28602, 75/136280, and 75/136279.
Japanese Patent Application No. 82/69471 demonstrates the use of a cationic polymer to assist in increasing the interlayer bond strength necessary for forming paper from pulp slurries. The cationic polymer being an acrylamide-dimethylaminoethylmethacrylate (DMAEM) sulfate copolymer. Japanese Patent Application No. 75/148242 discloses mixing a cationic amine salt having a 9:1 mole ratio and being a 30% aqueous solution with dimethylaminoethyl methacrylate methochloride polymer in a 86:14 ratio to provide a flocculant.
Japanese Patent Application No. 75/148241 prepares a cationic flocculant of acrylamide and DMAEM sulfate which are copolymered in a 9:1,8:2,7:3,5:5, or 3:7 mole ratio as a 20% aqueous solution. Japanese Patent Application No. 74/146371 also prepares a cationic flocculant by copolymering acrylaiide with a salt or quaternary derivative of DMAEM in aqueous acetone.
Although the aforementioned patents recognize that ionic flocculants may be used to assist in the dewatering of sludge, the present inventors have discovered that the particular cationic amine salt used in accordance with the process parameters of the present invention dramatically improves the capture of solids in the gravity drainage filtrate of a twin belt press with lower solids in the filtrate and a dryer cake at discharge. The cationic amine salt solution used in accordance with the process developed by the present inventors is much more cost effective than earlier flocculants, provides a cleaner filtrate requiring less recycle of total suspended solids therein, permits a wider polymer dosage, and allows for very fast floc formulation. Additional advantages of the present invention shall become apparent as described below.